Many people use mobile stations, such as cell phones, personal digital assistants (PDAs), tablet computers, laptop computers, desktop computers, in-car computers, and so on, to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are often referred to as “1×RTT networks” (or “1× networks” for short), which stands for “Single Carrier Radio Transmission Technology.” These networks typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Mobile stations typically conduct these wireless communications with one or more base transceiver stations (BTSs), each of which send communications to and receive communications from mobile stations over the air interface. Each BTS is in turn communicatively connected with an entity known as a base station controller (BSC), which (a) controls one or more BTSs and (b) acts as a conduit between the BTS(s) and one or more switches or gateways, such as a mobile switching center (MSC) and/or packet data serving node (PDSN), which may in turn interface with one or more signaling and/or transport networks.
As such, mobile stations can typically communicate with one or more endpoints over the one or more signaling and/or transport networks from inside one or more coverage areas (such as cells and/or sectors) of one or more BTSs, via the BTS(s), a BSC, and an MSC and/or PDSN. In typical arrangements, MSCs interface with the public switched telephone network (PSTN), while PDSNs interface with one or more core packet-data networks and/or the Internet.
To meet increasing demand for high-speed data on mobile devices, cellular service providers have begun implementing “4G” networks, which provide service under one or more 4G air interface protocols, such a long-term evolution (LTE) protocol. LTE was developed by the 3rd Generation Partnership Project (3GPP), and is based on GSM/EDGE and UMTS/HSPA network technology.
In the context of LTE, a mobile station is typically referred to as a “user entity” (UE), and may take various mobile and stationary forms, such as a mobile phone, tablet computer, laptop computer, desktop computer, or any other device configured for wireless communication. Herein, the terms “mobile station,” “wireless communication device” (or WCD), and “user entity” (or UE) may be used interchangeably.
Further, each coverage area in a cellular network may operate on one or more carriers each defining a respective downlink frequency range or “downlink channel” for carrying communications from the base station to UEs and a respective uplink frequency range or “uplink channel” for carrying communications from UEs to the base station. Further, both the downlink channel and uplink channel of each carrier may be divided into sub-channels for carrying particular communications, such as one or more control channels for carrying control signaling and one or more traffic channels for carrying application-layer data and other traffic.
In general, when a UE is positioned within coverage of a base station, the base station may serve the UE on a particular carrier and may allocate resources on that carrier for use to carry communications to and from the UE.
For instance, in a system operating according to an orthogonal frequency division multiple access (OFDMA) protocol, such as LTE, the air interface is divided over time into frames and sub-frames each defining two slots, and the uplink and downlink channels are divided over the bandwidth of the carrier into sub-carriers that are grouped within each slot into resource blocks. When a UE is positioned within coverage of a base station in such a system, the UE may register or “attach” with the base station, and the base station may then schedule particular downlink and uplink resource blocks on the air interface to carry data communications to and from the UE. Further, the base station and UE may modulate their air interface data communications at a coding rate selected based on quality of the UE's coverage, such as with higher rate coding rate when the UE is in better coverage of the base station and with a lower coding rate when the UE is in worse coverage of the base station.
With such an arrangement, the bandwidth of the carrier on which the base station serves a UE may define an effective limit on the rate of data communication between the base station and the UE, as the bandwidth would define only a limited number of resource blocks per slot, with data rate per resource block being further limited based on air interface conditions. By way of example, in accordance with the LTE standard, the uplink and downlink channels on each carrier may be 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, each resource block spans 180 kHz, and each slot is 0.5 milliseconds long. Accounting for guard bands at the edges of each carrier, the maximum number of resource blocks per 0.5 millisecond slot is thus 15 in 3 MHz, 25 in 5 MHz, 50 in 10 MHz, 75 in 15 MHz, and 100 in 10 MHz. Consequently, an LTE base station (interchangeably referred to as a an “eNodeB” herein) that serves UEs on such a carrier would have only the specified number of resource blocks available to allocate for air interface communication per slot, with coding rate in each resource block being further limited based on air interface conditions.
One way to help overcome this per-carrier data rate limitation is to have a base station serve a UE on multiple carriers at once, providing what is known as “carrier aggregation” service. With carrier-aggregation service, multiple carriers from either contiguous frequency bands or non-contiguous frequency bands are aggregated together as “component carriers” to increase the overall bandwidth available per slot by providing a greater number of resource blocks in which the eNodeB can schedule uplink and downlink communication. Further, where the concurrently-used component carriers are sufficiently distant from each other in the frequency spectrum, serving a UE concurrently on those component carriers may additionally create a frequency-diversity effect that could further improve data throughput.
In a further aspect of OFDMA protocols, such as LTE, reception at cell edges may be problematic for various reasons. For example, the greater distance to a base station at a cell edge may result in lower signal strength. Further, at a cell edge, interference levels from neighboring cells are likely to be higher, as the wireless communication device is generally closer to neighboring cells when at a cell edge.
In an effort to improve the quality of service at cell edges, 3GPP LTE-A Release 11 introduced a number of Coordinated Multipoint (CoMP) schemes. By implementing such CoMP schemes, a group or cluster of base stations may improve service at cell edges by coordinating transmission and/or reception in an effort to avoid inter-cell interference, and in some cases, to convert inter-cell interference into a usable signal that actually improves the quality of service that is provided.
LTE-A Release 11 defined a number of different CoMP schemes or modes for both the uplink (UL) and the downlink (DL). For the downlink, two basic types of CoMP modes are set forth: joint processing (JP) schemes and coordinated scheduling/beamforming (CSCH or DL-CSCH) schemes. For the uplink, numerous types of CoMP modes have been devised.
Uplink CoMP modes may involve interference rejection combining (IRC) or coordinated scheduling for purposes of reducing or preventing interference between transmissions from different user entities (UEs). Additionally or alternatively, various uplink CoMP modes may involve “joint reception” and/or “joint processing.” Joint reception generally involves multiple base stations receiving an uplink signal that is transmitted by a given UE. Joint processing generally involves the multiple base stations that received the uplink signal from the UE, sending the respectively received signals or a decoded and/or processed version of the respectively received signals to one another, or just to a master base station in the group, such that the multiple received versions of the UE's transmission can be combined to improve reception and/or reduce interference.
Various types of joint processing have been implemented on the uplink. For example, joint processing on the uplink can be centralized. When a centralized CoMP mode is implemented on the uplink, the coordinating base stations may simply pass the entire received signal from a given UE on to a master base station, which then uses the received signals from multiple base stations to decode and/or process the signal from the given UE. Joint processing on the uplink can also be de-centralized to varying degrees. Specifically, when a decentralized CoMP mode is implemented on the uplink, a coordinating base station may decode and/or process the received signal from a given UE, and then send the decoded and/or processed signal from the given UE to the master base station. The master base station can then combine or select from the decoded and/or processed versions of the UE's transmission, which are sent to the master base station from one or more coordinating base stations that receive the UE's signal (and possibly a version of the UE's signal that is received at the master base station itself).